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Tuesday, December 13, 2011

Solar Dryer extends harvest and creates microenterprise opportunity

Elyse Peterson is an experienced food scientist in the dairy, seafood, meat, and soft drink industries, Elyse served two terms in the Peace Corps helping promote sustainable food security solutions. As part of her work in the Peace Corps, she helped to develop a Solar Food Dryer which became the catalyst for a community-based food security and economic development project in in Antigua.

We talked to Elyse about this technology, which is highly appropriate for many tropical areas, and may have application into other climate zones:

Photo: Mangoes drying in the sun.

TBG: How does Solar Food Drying work?Solar drying is a low cost method of drying food. It is important to understand moisture in food and the properties of the air around us.

All food contains moisture which comes in three forms: liquid, solid & gas. This moisture is what microorganisms need to live and thrive, so in order to stop microorganism growth you may reduce this moisture to a safe level to preserve and extend the shelf-life of your harvest.

The design of the Solar Dryer harnesses the power of the sun's rays to raise temperatures within the unit to between 110 - 130 degrees Farenheit.

This heat lowers relative humidity while increasing absolute humidity, so that the air inside the unit attempts to reach the absolute humidity of the climate outside the unit by taking moisture from the food.

Coupled with proper air circulation, this is what makes the solar dryer work.

Photo: Solar dryer built with locally available materials in Antigua, 2007.

TGB: What kind of Solar Dryer Designs have you developed?

There are three basic designs of solar dryers that you may follow when building your own solar dryer: direct absorption, indirect heating, and mixed mode. In this project a direct absorption solar dryer was designed because it was found to be the most sustainable for Hawaii’s needs.

In these designs the food is placed inside a cabinet or “hot box” which allows the rays of the sun to heat up food and air around it. A compartment with a transparent roof and insulated walls is used, but if designed properly all the walls can be transparent. These may also use reflectors at the bottom of the compartment to increase light (metal or foil).

Ventilation holes are required to promote proper air circulation. Indirect heating dryers dry the food with heated air collected by a “solar panel”. The food is placed in an insulated heating chamber with proper air circulation.

These models are effective but cost a lot of money. Mixed Mode dryers are a combination of the other two designs. Food is heated directly by the sun but additional heat is collected with the “solar panel”. These can cost about $1300EC to build, effective but expensive.

Businesses attempting to expand and produce higher quality products should look into building one of these models (for the purposes of this project and the situation in Hawaii the direct absorption model is the best fit). When constructing your dryer be sure to follow the design carefully, because the angles achieved are vital for efficient processing.

Photo: Mangoes selected for drying are peeled and cut.

TGB: How do we select produce to dry?When selecting food to process in the solar dryer it is important to remember that drying will not improve the quality of the produce. Only produce that you would consume fresh should be dried.

Produce with cuts, bruises, or other evidence of contamination should not be used. Select fruit that is ripe yet firm. Using over-ripe fruit can cause the final product to come out brown and sticky. Ripeness is about 2/3 ripe.

Photo: Mango is prepared for dehydration in the Solar Dryer.

TGB: How do we prepare produce for drying?Produce shall be soaked in a bleach solution to remove microbial contamination (1 tsp bleach in 1 gallon water). Peel fruits and cut into appropriate sized pieces. The smaller the size piece the faster the drying time. As the size of the piece increases the time for drying grows exponentially.

Example: A slice or piece two times larger with take four times longer to dry. Experiment with your dryer to see what size piece is the best.

TGB: Doesn't that mean we will be eating bleach?

There are alternative methods for sanitation, but bleach is the most available and inexpensive. One must remember that bleach is highly volatile. It evaporates into the air, so there is essentially no bleach on the product by the time we eat it. This is a standard practice in the food industry that many of us don't know about but owe our safety to.

TGB: Does it work in the rain?How long does it take?No. The dryer should only be used on days with consistent powerful sun as to reduce the time required for drying. Depending on the results you may find it necessary to rotate the trays throughout the day so every tray gets equal amounts of direct sun exposure.

Drying should take about 12 hours of full sun power (possibly one day with good sun). Optimal temperature for solid drying is 110-120°F but 130°F will be the most effective temperature. Putting a thermometer in the dryer during processing is a safe way of monitoring the efficiency of your dryer.

Keep a close eye on the produce towards the end of drying because drying happens at a faster rate just before it’s reached 10% moisture. To test for doneness you should see that vegetables at about 10% moisture will be brittle and easily can be broken apart.

Fruits should be soft and chewy, but test the moisture content to verify 5-8% moisture (refer to Principles of Solar Drying). There are a few quality issues that need to be considered when solar drying produce.

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For more information and detailed instructions on how to build your own Solar Food Dryerin Hawaii, download the Solar Dryer Manual here:

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